The present invention relates to stress sensors in general. In particular, it considers the construction of a stress sensor and a method for adapting the instrument for imbedding into a material in which knowledge of the stress changes is desired.
Many environments present changing stresses that are of interest. For example, in mines it is important to monitor changes in stress in rock formations to determine possible impending failures.
Studies have shown that there is a maximum value of stress concentration factor for a circular rigid inclusion of approximately 1.5 times the stress applied to the body. This maximum is approached rapidly when the ratio of the effective Young's modulus of the inclusion to Young's modulus of the host material is greater than four. When the inclusion is this stiff, the stress induced in it is largely independent of the strain of the host material and the inclusion acts as a stress meter.
The stress in ice is of interest, for example, in areas where petroleum drilling platforms are to be used. For example, stresses in an ice sheet are important in platform design as they affect ice loads on the platform. Changes in the stress field, if known and if sufficiently serious, can allow for accommodation or corrective action.
Estimates of ice stresses obtained by considering the failure strength of sea ice may be too conservative in many applications. For example, in landfast ice areas, stresses may be smaller due to grounded pressure ridges and rubble piles, imperfections, and stress concentrations in the ice sheet. Also, landfast ice areas may be protected from the pack ice by the shear zone and barrier islands.
In addition, it is desirable to monitor ice stress to understand how the stresses relate to ice movement. Knowledge of stress near a structure, far field stress, and vertical stress gradients in an ice sheet may be of interest.
It is important in such monitoring to have the monitors reliable, simple, and without large power demands. The monitor signals should be easily transmitted. It is desirable also to have a gauge that is linearly responsive, accurate, responds immediately to applied loads, affords a means for determining the direction of applied stress, insensitive to creep deformation, and insensitive to temperature changes and differential thermal expansion between the host material and the gauge.
An existing stress sensor is described in U.S. Pat. No. 4,159,641 to Hawkes. This stress sensor uses a vibrating wire excited to vibrate at its natural frequency by a strong magnetic field. The natural frequency of the wire is a function of strain on the wire. An operator controls the frequency of the field and finds a "plucking" frequency at about the natural frequency of the wire. The device has a three-pole electromagnet constituted of a yoke and a permanent magnet. The yoke provides two poles of the same polarity, say South, and the permanent magnet provides a single pole of opposite polarity, say North. As induced magnetic field in the wire has corresponding poles of opposite polarity. The frequency of the modulated magnetic field can be varied with a known control. The coil and magnet also detect wire vibration. The permanent magnet induces a local magnetic field in the wire. As the wire moves, it appears as a changing magnetic field to the coil, inducing a voltage in it. This voltage has a frequency corresponding to the frequency of the vibrating wire, and accordingly the frequency of the wire is directly determined as a voltage signal. In this design, because the coil and magnet assembly is used for both plucking and signal detection, wire excitation must be followed by a period of "listening" for the return signal. Known digital logic circuitry can be used to count and display the frequencies.
A different stress sensor has a thin profile and a diaphragm. The cell can be circular or rectangular. The diaphragm type of cell endeavors to provide an instrument that is flat and stiff relative to the host to be studied. This cell attempts to avoid a high "aspect ratio", which is thought to produce greater stress at the cell than the stress in the far field. This design recognizes that the stiffer diaphragm cell, the less sensitive the cell is to changes in the Young's modulus of the host.
Another type of strain measuring device deforms with the host material without reinforcing it in any way. This device measures the strain in the host by relative movement of two fixed points on the cell.
A photoelastic type of stress indicating instrument ascertains the magnitudes of the primary and secondary principal stresses in a biaxial stress field from isochromatic fringe patterns in a hollow, cylindrical stress meter. The photoelastic stress meter consists of a hollow core glass cylinder bounded around its periphery in a hole drilled or cast in the host being studied. The stresses in the body are transmitted to the meter where they are revealed as isochromatic fringe patterns when the meter is viewed with polarized light. The fringe pattern gives the directions, signs, and magnitudes of the two principal stress components in the plane of the meter.
The stress sensor described in U.S. Pat. No. 4,159,641 has a partially cylindrical body with one side milled flat. A platen and wedge cooperate with the flat to secure the gauge into the body being studied. The overall length of the platen is small, relative to the sensing zone of the instrument. This type of instrument requires an open bore hole to be maintained in the host material and cannot be completely imbedded. Imbedding produces anomalous stress concentrations in the ends of the instrument that induce errors in the instrument's read-out. The requirement of the wedge assembly to secure the instrument into the body being studied results in the inability of the gauge to be used in material exhibiting extensive time dependent deformation, creep, such as ice. This inability results from the prestressing required by the wedge assembly. The problems associated with an open bore hole and creep also occur with photoelastic sensors.